In web rotary printing presses, a web is cut in a center position of a margin portion between printed images in a direction orthogonal to a web transport direction by a cut-off cylinder in a folding machine and thereafter folded and delivered as a signature. However, during an acceleration (hereinafter, termed as “speed acceleration”) of a web rotary printing press, due to a change in stretch or tension or the like of the web, the position where the web is to be cut by the cut-off cylinder is shifted with respect to the positions of printed images on the web, and the web may not be cut in the center position of the margin portion in some cases. In such cases, the printed images may appear in shifted positions when the signature is opened, or the web may be cut in an image portion in the worst cases.
For this reason, a cut-off register control device has been conventionally provided for the web to be cut in an accurate position between images by the cut-off cylinder. Such a cut-off register control device is configured to allow the web to be always cut in an accurate position by the following manner. The cut-off register control device prints a register mark in a margin portion at a lateral side of a position corresponding to an image on the web, simultaneously with the image, or uses a characteristic portion in a printed image as a register mark and then detects the register mark by a detector provided to an entrance of the folding machine (where the former is located). The cut-off register control device compares a rotation phase of the web rotary printing press when the register mark is actually detected with a reference rotation phase of the web rotary printing press for the web to be cut in a correct position and then adjusts the length of a transport path for the web by the amount corresponding to the gap by moving the position of a compensator roller provided upstream of the detector, the transport path extending to the position where the web enters the folding machine <refer to Patent Literature 1>.